At the very beginning of aviation, dead reckoning navigation utilized astronomical sightings or terrestrial features as waypoints for correcting & updating aerial platform position. Initially, in terrestrial feature or ground point positioning the pilot or navigator sighted the object & flew over it at the desired heading noting the time in the navigational log. This action of sighting, correcting course & physically transiting the ground point at a noted time & measured offset (vertical & horizontal) constituted the navigational fix or update. Accuracy was limited by many factors and was superseded by LORAN (LOng RAnge Navigation) which in its final implementation (LORAN-C) had location accuracies of 50 meters ([m]) but required ˜20 permanent ground transmitting stations arrayed across the continental US (CONUS) to service CONUS air traffic. Updated but undeployed versions of LORAN (e-LORAN) promise greater accuracies (8 [m]) but still require numerous ground based transmitting stations over the service area to attain maximum accuracy. So while LORAN & other terrestrial radio based positioning systems can be used beyond the fringes of their ground based networks their accuracy diminishes as the distance increases and the transmitter directional diversity decreases. For example, the OMEGA radio positioning system had worldwide coverage using 9 ground transmitters but its positional accuracy was only 2.2 kilometers ([km]).
Ground based radio navigation positioning systems (radio nav) have been largely replaced by global navigation satellite systems (GNSS) such as the global positioning system (GPS) or its Russian equivalent GLONASS. Civilian GPS operating autonomously typically have accuracies in the 10-20 [m] range unless they have been initialized for a least 15 min. by remaining motionless with unobstructed sky view to 5 degrees above the horizon. At this point, sub meter (<˜10 cm) accuracy is obtainable with roving GPS receivers by adding their received coarse acquisition (CA) signal to an offset signal transmitted by the now motionless, initialized base station. The wide area augmentation system (WAAS) of the Federal Aviation Administration is a CONUS wide version of this concept. In this case ground reference stations receive satellite GPS signals, calculate the offset from the CA signal & uplink the corrections to WAAS dedicated satellites which then transmit the correction to commercial airplanes providing higher accuracy positional data (7.6 [m] or better). To achieve this performance, with high reliability, ˜20 permanent ground based reference stations distributed across CONUS are required. The US Coast Guard implemented a similar system (DGPS or differential GPS) with higher accuracies (˜10 cm) but requiring a higher concentration of ground stations (˜60 ground stations to cover CONUS navigable waterways).
GPS denial is always a possibility. GPS satellites broadcasting 25 [Watts] at ˜11,000 [mile] range can be swamped by readily available, equal power jammers operating at much closer range (<10-100 [mi]). Better GPS signal conditioning, directional antennas, & deeply integrated GPS/INS flight computers can somewhat ameliorate this ˜40-60 [dB] jammer advantage. Examples of civilian GPS being jammed are numerous, the former LightSquared being the poster child; their network base stations interfered with GPS receivers at distances from 600 [ft.] to 185 miles. Reports of significant interference with US military GPS have also been reported with the US drone diverted by the Iranians being the most prominent. So it is desirable for military systems to have the ability to augment GPS capabilities in GPS compromised situations. It is also desirable that the more vulnerable civilian aircraft also have the ability to accurately navigate in the face of intentional or unintentional GPS interference.
Inertial navigation systems (INS) are immune from the same sort of interference GPS is susceptible to. Over the last 20 years, significant advances in size, weight, durability, & power consumption have been made but not so much in accuracy. Their drift as a function of time is characterized by FIG. 1 which shows the performance bands for 3 classes of systems, low, moderate, & high cost. At times <<˜1-2 [min] their accuracy is limited by scale factor stability and is characterized by a positional error that increases linearly in time while for times >>˜1-2 min bias stability dominates with integrated positional error increasing quadraticly with time. This rapid deterioration renders all INS systems incapable of even rough navigation (<200 [m] CEP) without positional updates at 1 [hr.] or more frequent intervals. For precise munition delivery (<=2 [m] net CEP) and assuming the munitions' INS system is positionaly updated 1 min before impact, a low cost (10 k$) INS (CEP(1 min)˜40 [m]) is unusable, a moderate cost (100 k$) INS (CEP(1 min)=1-2 [m]) is a possibility, while a high cost (>750 k$) system (CEP(1 min)˜0.4 [m]) is too expensive. High cost (>750 k$) systems would not be used in munitions but on manned (military or civilian) aircraft & other strategic platforms.
For ground based vehicles, in addition to intentional or unintentional electromagnetic interference of GPS, steep local terrain can obscure GPS satellites resulting in a loss of positioning function (fewer than 3 satellites visible). Even the most sophisticated ground based vehicles do not have INS systems so this loss of function is without recourse.
So given the ability to deny GPS coverage, the difficulty of utilizing ground based radio transmitters for navigation over denied territory, & the short time frame of accurate INS capabilities it would be desirable to have a system & method for securely, quickly, & reliably updating the flight navigation solution systems in an autonomous fashion. It is also desirable to have a system & method for determining the accuracy & reliability of navigation subsystems comprising the flight navigation system. It is also desirable to have a ground navigation system that can function in the absence of GPS.